1. Field of the Invention
This invention relates to a pneumatic radial tire, and more particularly to a pneumatic radial tire in which the weight reduction of the tire is realized by a reinforcing structure using a monofilament cord as a cord constituting a belt layer and the degradation of belt durability is advantageously controlled by rationalizing a rubber gauge between the monofilament cords located in the adjacent belt layers.
2. Description of Related Art
In the pneumatic radial tire, the rigidity of the belt is considerably increased for improving so-called hoop effect, so that the weight is generally heavier than that of a pneumatic bias tire. As a result, the degradation of the fuel consumption become a problem. In the radial tire, therefore, various attempts are made for solving such a problem to reduce the tire weight.
For example, as shown in FIG. 3a, there is widely used a belt layer 21 formed by rubberizing metal twisted cords or organic fiber twisted cords as a cord of the belt layer from the old time. Considering a case that the belt layer 21 is replaced with a belt layer 22 formed by rubberizing monofilament cords and having the same elastic modulus as in the belt layer 21 within a usable region thereof as shown in FIG. 3b, a diameter d2 of the monofilament cord can be made smaller than a diameter d1 of the twisted cord and hence the thickness of the belt layer constituted by the cords and the coating rubber can be decreased, so that the weight reduction of the tire can be attained.
Moreover, the term xe2x80x9cmonofilament cordxe2x80x9d used herein means a cord consisting of a single filament without twisting.
However, if it is intended to reduce the tire weight by merely changing the design from the belt layer 21 into the belt layer 22 as mentioned above, the flexible followability of the cord to the coating rubber in case of using the twisted cord is not realized in the deformation of the belt layer using the monofilament cords, so that the shearing strain between the belt layers becomes large and the separation failure between the monofilament cord and the coating rubber is apt to be easily caused at both side end portions of the belt layer by heat generation resulted from such a shearing strain and hence there is a drawback that the durability of the belt lowers.
Therefore, even if the weight reduction of the tire is attempted by using the monofilament cord instead of the twisted cord as a cord of the belt layer as mentioned above, it is strongly demanded to develop a new technique capable of ensuring the sufficient belt durability.
It is, therefore, an object of the invention to solve the above problem for satisfying such a demand, and to provide a pneumatic radial tire capable of realizing the weight reduction of the tire and effectively controlling the degradation of the belt durability by using a monofilament cord as a cord constituting the belt layer.
According to the invention, there is the provision of in a pneumatic radial tire comprising an innerliner, a carcass toroidally extending between a pair of bead portions and externally contacting with the innerliner, a belt arranged at an outer circumference side of a crown portion of the carcass, one or more cap layers covering the belt over substantially a full width thereof, and a tread arranged at an outer circumference side of the cap layer, an improvement wherein the belt is comprised of at least two belt layers each formed by covering metal monofilament cords or organic fiber monofilament cords with a coating rubber, and a rubber gauge between the monofilament cords located in the adjacent belt layers is made 1.5-5 times of a diameter of the monofilament cord.
In the tire according to the invention, the weight reduction of the tire can be attained by constituting the belt layer with the metal monofilament cords or organic fiber monofilament cords. And also, the rubber gauge between the monofilament cords in the adjacent belt layers is made 1.5-5 times, preferably 2.5-5 times of the diameter of the monofilament cord, so that the rubber gauge between the cords is sufficiently ensured while keeping the above effect of reducing the tire weight, and also the occurrence of the separation failure between the monofilament cord and the coating rubber at both side edge portions of the belt layer based on the heat generation resulted from the shearing strain between the belt layers can be prevented in the deformation of the belt layer, whereby the degradation of the belt durability can effectively be controlled.
When the rubber gauge between the monofilament cords located in the adjacent belt layers exceeds 5 times of the diameter of the monofilament cord, the sufficient weight reduction can not be achieved with the increase of the rubber gauge, while when it is less than 1.5 times, the effect for dispersing the shearing strain between the belt layers by the coating rubber is hardly obtained, and as a result, once the separation occurs between the monofilament cord and the coating rubber at both side end portions of the belt layer, the progress of the separation is fast and the early degradation of the belt durability is worried and hence the sufficient belt durability can not be ensured.
When the belt layer is constituted by arranging a plurality of cord groups, each of which groups being comprised of plural monofilament cords arranged side by side, in parallel to each other, for example, when a plurality of cord groups each comprised of 3-5 straight monofilament cords having a circular section are arranged in parallel to each other, a distance between the cord groups in the belt layer can sufficiently be ensured, so that even if the separation once occurs between the monofilament cord and the coating rubber at both side edge portions of the belt layer, the progress of the separation can be controlled.
In addition to the weight reduction by the belt as mentioned above, it is advantageous to further promote the weight reduction as a whole of the tire by reducing the weight of the innerliner. Concretely, the thickness of the innerliner is preferable to be made a range of 0.15-0.8 mm as a range enabling both the reservation of a sufficient air tightness and the achievement of the weight reduction of the tire.
The carcass is preferable to be comprised of at least one carcass ply containing rubberized polyethylene naphthalate cords therein.
When the weight reduction of the tire is realized by changing design from the belt layer using the twisted cord into the belt layer using the monofilament cord and having the same elastic modulus as in the former belt layer within the usable region thereof and the degradation of the belt durability is advantageously controlled by rationalizing the rubber gauge between the monofilament cords located in the adjacent belt layers, there is a tendency of lowering a breaking tenacity of the belt. In such a pneumatic radial tire, however, when the carcass is comprised of at least one carcass ply containing high-rigidity rubberized poly-ethylene naphthalate cords therein, the lowering of the breaking tenacity of the belt -can be compensated by the increase of the breaking tenacity of the carcass to mitigate the tension bearing ratio of the belt.
And also, when the belt is reinforced with a cap layer of rubberized polyethylene naphthalate cords extending substantially in a circumferential direction of the tire, the cap layer can sufficiently bear bending rigidity in the deformation of the belt and the like as compared with a case of applying a usually used cap layer of nylon cords, whereby heat generation resulted from the shearing strain between the belt layers is controlled to advantageously prevent the separation failure between the monofilament cord and the coating rubber at both side edge portions of the belt layer and hence the degradation of the belt durability can be avoided.
Furthermore, the lowering of the breaking tenacity of the belt as a weak point when the belt layer using the monofilament cords is used instead of the belt layer using the twisted cords is sufficiently overcome by the synergistic action of the above construction of the cap layer and the above construction of the carcass using the polyethylene naphthalate cords, whereby the weight reduction of the tire can be more manifested.
In addition, when a width of the tread is within a range of 70-80% of a tire maximum width, an excellent steering stability can be ensured while controlling the increase of the tire weight.
That is, when the tread width is less than 70% of the tire maximum width, the ground contact area is decreased and it is obliged to degrade the steering stability, while when it exceeds 80%, the weight of the tread rubber increases and hence it is unavoidable to increase the tire weight.
In the above tire, it is further preferable to reduce the weight of the sidewall portion by decreasing the rubber gauge of the sidewall portion. In this case, in order to compensate for the lowering of the stiffness resulted from the decrease of the rubber gauge of the sidewall portion, a bias insert comprised of organic fiber cords extending at a cord angle within a range of 30-60xc2x0 with respect to a phantom radial line segment is arranged between a main body and a turnup portion in the carcass so as to extend between a position near to the bead core and a position located outward from the position of the tire maximum width in the radial direction. The bias insert advantageously enhances the longitudinal stiffness of the tire to bring about the improvement of the steering stability.
Particularly, when the cord angle of the organic fiber cord is about 45xc2x0 with respect to the phantom radial line segment, the longitudinal stiffness becomes remarkably high. When the cord angle is less than 30xc2x0 or exceeds 60xc2x0, the increase of the longitudinal stiffness becomes small.
Moreover, the extension of the bias insert between the position near to the bead core and the position located outward from the position of the tire maximum width in the radial direction is favorable to be effective in the increase of the longitudinal stiffness. However, when the extending region of the bias insert in the radial direction is made narrower than the above, the effect of increasing the longitudinal stiffness becomes smaller.
Furthermore, when a circumferential insert of organic fiber cords extending in the circumferential direction of the tire is arranged in at least a part of a region between a position near to the bead core and a position near to a side edge of the belt instead of the bias insert or in addition to the bias insert, the lateral stiffness of the tire can be increased, while the ride comfort against vibrations can be enhanced while controlling the increase of the radial stiffness.
Moreover, the circumferential insert is preferable to be arranged between the vicinity of the bead core and the vicinity of the position of the tire maximum width in a width in the radial direction of no less than 10 mm in order to bring about a sufficient reinforcing effect by the circumferential insert.